Typical fuel bundle assemblies in boiling water nuclear reactors include a plurality of elongated fuel rods supported between upper and lower tie plates. The fuel rods pass through a plurality of fuel rod spacers which provide intermediate support to retain the elongated rods in spaced relation and to restrain the rods from lateral vibration.
Each of the fuel rods comprises an elongated tube containing the fissile fuel (such as uranium or plutonium dioxide) in the form of pellets, particles, powder or the like, sealed in the tube by upper and lower end plugs. Most of the lower end plugs are formed with a taper for registration and support within cavities formed in the lower tie plate, while corresponding upper end plugs are formed with extensions which register with support cavities formed in the upper tie plate. These full length rods are spring biased towards the lower tie plate so as to prevent undesirable axial movement of the rods. It would be desirable to eliminate or reduce the number of parts associated with the upper tie plate while at the same time eliminating concerns about coolant flow impact on the conventional protruding lower tie plate end plugs.
Each fuel bundle assembly in current designs also contains several fuel rods which have threaded lower end plugs. These would typically include eight tie rods and as many as fourteen partial length fuel rods (PLR's) which terminate short of the upper tie plate. Fuel bundles with PLR's are disclosed, for example, in commonly owned U.S. Pat. Nos. 5,112,570 and 5,017,332. Fuel bundles including fuel rods serving as tie rods with threaded upper and lower end plugs are disclosed, for example, in commonly owned U.S. Pat. Nos. 4,357,298 and 4,420,458. After irradiation, it has been found that many of the threaded rods are not removable without extreme measures which are time consuming and which can damage the rods beyond repair. The apparent cause of the problem is that the lower end plugs of the rods stick in the lower tie plate as the result of corrosion which occurs on the male threads of the Zircaloy end plugs. It will be appreciated that increasing the number of fuel rods which are threadably connected to the lower tie plate will increase the chances of experiencing fuel rod sticking. Thus, there is a need for a simple and cost effective means for minimizing or eliminating the problem.